July 3, 2006

Memory impairment in bipolar disorder

Sources of declarative memory impairment in bipolar disorder: Mnemonic processes and clinical features

What are declarative memories?

Declarative memories are our conscious memories, also called explicit memories. They can be episodic, a memory of a specific event, or semantic, a memory of facts and information. But they all require three distinct abilities, encoding, storing, and retrieving. Encoding is the ability to form a neurological pathway out of the information/event perceived. Storing is simply maintaining that neurological pathway for future retrieval, which is the ability to recover and utilize that information, the memory. The brain regions associated with these three abilities are the medial temporal lobe, and the prefrontal cortex.

Importance of this study:

There is some evidence that provides a connection between bipolar disorder and declarative memory impairments (specifically verbal declarative memories). Studies even concluded brain region abnormalities found in bipolar patients correlated to the regions utilized in the declarative memory process.

The article outlines very inconsistent findings for declarative memory impairments and the suspected source in bipolar patients. Some studies found it to be state specific (manic versus depressed) and correlating to unipolar depression. This would mean the impairments on declarative memories are greater in depressive states. Other research found no evidence for state specific impairments, and instead found severity of illness, relating to number of manic and depressive states, age of onset, and length of hospitalizations to be correlating factor. One study even found a higher cognitive dysfunction in males versus females. Needless to say, the results didn't paint a clear picture on the subject at hand.

These findings only prove the need for further more extensive research in this area. Many previous experiments made little effort in distinguishing manic/depressive states, and symptomatic versus euthymic (normal, not manic or depressed) at the time of study. These distinctions can have a very great impact on results and interpretations.

In an effort to surmount the previous limitations of the research, this study utilized the California verbal learning test, which allows for analysis of memory processes. They utilized 5 trails of word lists, which could be semantically organized for easier memory formation. These lists were administered and recalled either with short/long delay, and free or cued recall.

The participants used were 49 adult bipolar patients and 38 control subjects. In order to control for other variables, these bipolar participants were well documented on mood state, co-morbidities, medication usage, family history, cognitive functioning, and motor processing speed, at time of the study. The control subjects were demographically matched to the bipolar participants. Results were obtained using statistical analyses to analyze the differences in memory performance and general cognitive performance.

Results:

Because no significant differences in processing speed were found, any differences could be attributed to higher-level cognitive processing versus general impairments or motor slowing. Education was also found to not be a significant factor in the results.

On results of memory task performances, the bipolar group remembered significantly fewer words when compared with the control. These results were more profound in the later trails, which may signify a lesser repeated exposure effect. The significantly fewer words recalled for bipolar participants was found in all recall measures, independent of length of delay, and type of recall (cued or free). But they did not differ in their rate of forgetting; number of words retained from the original words recalled for both short and long delayed recall. They did however find no significant differences in organizational strategies, which included semantic clustering and serial positioning. But the bipolar group made more errors by recalling more words not actually on the original list.

The bipolar participant’s medications ranged from no medications, mood stabilizers or antidepressants, and 2 or more psychotropic drugs. After analyzing the effects of medication on their performances, no memory differences were found.

Correlations were found in those bipolar participants with mood disorder family history, and female gender. A positive family history and female gender were associated with higher total learning scores. No correlation was found for severity of symptoms at time of study.

Conclusions:

This study found that bipolar patients show declarative memory impairments, which are are present in both acute and remitted patients, and independent of their current state (manic versus depressive). These impairments were not a result of slowed motor or processing functions, and were not a result of inability to utilize strategic and organizational memory skills (such as clustering).

Opposing the researcher’s predictions, there were no significant differences found based on the duration of participant’s bipolar illness. But certain demographics not often examined in previous research on the subject, such as female gender and family history of mood disorders, were correlated with better performances. Although this study did not find a significantly higher score for females in the control group, some gender studies associate higher verbal acuity with “normal” (those lacking diagnosis of mental illness) females versus males. Few studies are in agreement with the results that a positive family history of mood disorders correlate with better intellectual functioning for those suffering with bipolar, and the underlying mechanisms for this outcome require further research.

Because there was no difference in rate of forgetting between groups, the authors concluded the area of impairment for bipolar patients was in the encoding process. Once the memory was formed, it was not forgotten at a higher rate, but there was a lower rate of initial memory formation/encoding. The bipolar group did have an increased number of error words recalled (words not in the list), which may be a result of a deficit in source monitoring, associated with the prefrontal cortex (an area of abnormality in bipolar patients).

Accompanied with its rigorous attempt at eliminating much of the constraints in previous research on this subject, this study has many of its own limitations. The overall sample size is fairly small, and may not properly reflect the population as a whole. This may have particularly affected analyses of subgroups and decreased statistical power for determining differences among those subgroups. The sample also included very few bipolar participants in a euthymic (normal, not manic or depressive) state. Further research accounting for these sample size limitations would be helpful.

Take home message:

This study found that declarative memory impairments may be attributed to a “trait abnormality of bipolar disorder”, and not to current mood state, type of medication, comorbidity of other psychiatric disorders. Further research is still necessary to support this data, as well as provide longitudinal evidence for possible differences in state and trait deficits within subjects.